GB2064895A

GB2064895A - Circuit for protecting storage cells

Info

Publication number: GB2064895A
Application number: GB8038586A
Authority: GB
Original assignee: Brown Boveri und Cie AG Germany; BBC Brown Boveri France SA
Current assignee: BBC Brown Boveri AG Germany; BBC Brown Boveri France SA
Priority date: 1979-12-04
Filing date: 1980-12-02
Publication date: 1981-06-17
Also published as: DE2948700C2; GB2064895B; FR2471685A1; DE2948700A1; JPS5691378A; FR2471685B1; US4349773A

Description

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GB 2 064 895 A .DTD:

SPECIFICATION .DTD:

Circuit for protecting storage cells The present Application relates to a protective circuit for an electrochemical storage unit, which unit comprises an electrochemical storage cell or a plurality of such cells arranged in parallel and is connected in series with other such units to form a battery. More particularly, the cells are based on alkali metal and chalcogen with at least one anode chamberfor the reception of the anolite and one cathode chamber for the reception of the catholite, said chambers being separated from each other by an alkali-ion conducting solid electrolyte wall. 10 Rechargeable electrochemical storage units of the above kind, having cells equipped with solid electrolytes, are very suitable for the construction of accumulators of high energy and power density.

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However, such a unit cannot either be over charged or overdischarged, as is possible in the case of lead accumulators. For this reason, the total capacity of a battery consisting of a series arrangement of such units is determined by the storage unit having the smallest capacity. In particular, a storage unit which is used with 15 a charging condition which differs from that of the other storage units of the battery can never be synchronized with the other storage units.

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A further drawback occurs when the battery discharges through a load. If one of the storage units of the battery has already discharged, the discharge current of other units in the battery act on the already discharged units as an externally applied current. As a result of this, the cells of the already discharged unit 20 may be discharged further and then destroyed.

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It has been proposed to overcome this problem by the provision of a protective circuit for each storage unit which acts to bypass the unit in response to excessive charging or discharging of the unit, thereby allowing the remaining unit battery to continue being charged, or to continue discharging through an external load, without damage to the unit. However, the disadvantage of this proposal is that matching to the threshold 25 voltage and internal resistance of the storage unit is often possible only by using several components in the bypass paths. Furthermore, the components proposed for use in the protective circuit are temperature sensitive, so that direct incorporation of the circuit in the battery is impossible.

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The present invention seeks to mitigate at least some of the above disadvantages.

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According to the present invention there is provided a protective circuit for an electrochemical storage unit, which unit comprises an electrochemical storage cell or a plurality of such cells arranged in parallel and is connected in series with other such units to form a battery, wherein the protective circuit provides a protective element connected in parallel to said storage unit to by-pass said unit in response to excessive charging or discharging of the unit, wherein said protective element comprises a vessel filled with a decomposable and readily recombinable chemical compound, and an electrically conductive member extending into said vessel by an aperture in the vessel, and wherein the external boundary surface of said vessel forms an anode and said conductive member forms a cathode, the vessel and the conductive member being electrically insulated from one another.

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Preferably, the chemical compound in the vessel is a sodium hydroxide melt, the advantage of this chemical compound being its ready recombinability. Preferably, the vessel forming the protective element is 40 made of stainless steel, and nickel has been found to be particularly suitable. It is important to have a corrosion resistant material since the sodium hydroxide melt is extremely corrosive.

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Advantageously, the vessel is constructed as a totally enclosed sleeve and by means of a thermally conductive connection, the vessel may control a switch connecting one electrode of the electrochemical storage unit to a positive or negative terminal. The switch may be actuated in a simple manner by the heat 45 dissipated so that the circuit through the electrochemical storage unit is interrupted. The electrically conductive member is preferably a rod of a corrosion resistant, electrically conductive material, more particularly copper. Since part of the rod projects into the interior of the vessel and is in contact with the sodium hydroxide melt, the high corrosion resistant nature of copper is particularly suitable.

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Advantageously, the length of the rod is selected so that in the interior of the vessel it almost touchesthe 50 bottom. The rod projects from the vessel so that it can be easily connected to the negative electrical terminal.

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Advantageously, the operating limits of the protective element may be adapted to the maximum charging voltage and permissable discharging voltage of the storage unit. The protective element is temperature resistant particularly when constructed of nickel which has a melting point of 1453 C, and when a sodium hydroxide melt, which commences melting at 318 C is disposed in the interior of the vessel. If necessary, the 55 protective element together with the remaining components of the circuit may be disposed within the battery, Furthermore, since nickel and sodium hydroxide are not expensive, the cost of producing a protective element is relatively low.

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The invention will be described further by way of example, with reference to the accompanying drawings, in which Figure 1 is a circuit with a protective element associated with a unit of three parallel connected storage cells, Figure 2 shows the same circuit as Figure 1 with a unit of one storage cell, Figure 3 shows a modification of the circuit illustrated in Figure 1, Figure 4 shows the idealized current-voltage characteristic of a storage cell.

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2 GB 2 064 895 A 2 The circuit illustrated in Figurt) 1 comprises a protective element 1, a switch 2, a thermally conductive connection 3, a unit of three electro- chemical storage cells 4, a negative connecting terminal 6 and a positive connecting terminal 5. The storage cells 4 are connected in parallel. The unit can be combined in the form of a series connection with other such units into a battery.

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The protective element incorporated in the circuit is formed by a vessel which is filled with a chemical compound. The vessel is constructed as a sleeve, closed on all sides and is made of stainless steel, more particularly nickel. The vessel 1 contains sodium hydroxide melt. At one end the vessel 1 is provided with an aperture into which a rod 1 K is inserted. This rod consists of a corrosion resistant material more particularly copper, and is inserted into the aperture in electrically insulated manner so that the interior of the vessel is completely sealed with respect to the exterior. The first end of the said rod 1 K extends into the vessel so that it almost touches the bottom of the vessel. The second end of the rod projects beyond the vessel by a few millimetres. The external surfaces of the vessel 1 form the anode while the rod 1 K performs the function of the cathode. The anode of the protective element is connected to the positive connecting terminal 5 while the cathode 1 K is connected to the negative connecting terminal 6. Additionally, the protective element is connected to the switch 2 via the thermally conductive connection 3. Advantageously, the said switch is 15 constructed as a thermal switch. In this embodiment, the switch 2 is connected to the negative electric connecting thermal 6. The electric contact of the switch 2 is connected to a contact 8 to which the negative electrode of the storage unit is connected. The positive electrode of the storage unit is connected to the positive electric connecting terminal 5. An additional contact 9 is connected to the positive electrode of the unit and to the connecting terminal 5. The electric contact of the switch 2 can also be connected to the contact 9. With the battery in its normal operating state, the switch 2 is always connected to the contact 8.

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Due to the action of heat, the electric contact of the switch 2 can become detached from the contact 8 and can be connected to the contact 9.

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In Figure 2, the circuit again comprises a protective element 1, a switch 2, a thermally conductive connection 3 and a unit having a single electrochemical storage cell 4. The protective element 1 is constructed in the same manner as the protective element shown in Figure 1. The external boundary surfaces associated with the protective element 1 and functioning as anode are connected to the positive electric connecting terminal 5. The cathode 1 K of the protective element 1 is connected to the negative electric connecting terminal 6. The protective element 1 is connected to the switch 2 via the thermally conductive connection 3. The electric contact of the switch 2 is connected to a contact 8 to which the negative 30 electrode of the storage unit is also connected. The positive electrode of the storage unit is connected to the positive electric connecting terminal 5. A further contact 9 is connected to the aforementioned connecting terminal 5 and to the positive electrode of the storage unit. As in the exemplified embodiment shown in Figure 1, in normal operation the electric contact of the switch 2 is connected to the contact 8.

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As shown in Figure 3, the negative electrode of the storage unit can also be connected directly to the negative connecting terminal 6. In this exemplified embodiment the positive electrode of the storage unit is connected via the switch 3 to the positive connecting terminal 5. The contact 9 of this embodiment as illustrated in Figures 1 and 2 is connected to the negative electrode of the storage unit and to the negative connecting terminal 6. The external boundary surface of the protective element 1 of this embodiment is again connected to the positive element 1 of this embodiment is again connected to the positive electric 40 connecting terminal 5. The protective element 1 in this case is also constructed as a vessel containing a sodium hydroxide melt. The cathode 1 K of the protective element 1 in this case is also connected to the negative electric connecting terminal 6. The protective element is connected to the switch 2 via the thermally conductive connection 3. The positive electrode of this storage unit is connected to the contact 8 to which the switch 2 is connected in normal operation.

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The method of operation of the protective circuit shown in Figures 1 to 3 will now be described. The _ idealized current-voltage characteristic of the storage cell 4, illustrated in Figure 4, serves to provide a better understanding of the method of operation of the protective circuit and of the storage cells. The protective element 1 has a characteristic such as that of the storage cells 4. Reference will be made to the arrangements illustrated in Figures 1 and 3. All statements however, also apply to the arrangement illustrated in Figure 2. 56 .DTD:

To charge the unit of three parallel connected storage cells 4, a current source (not shown) is connected to the connecting terminals 5 and 6. The said current source supplies the negative charging current h illustrated in Figure 4. When all storage cells have reached the voltage designated with A in Figure 4, they will have been charged to their maximum capacity. An equalizing current, which ensures a uniform state of charge for all storage cells 4, flows between a plurality of parallel connected connected storage cells 4 of a unit. As a 55 result of charging, the storage cells 4 have acquired a high resistance, i.e. the current flowing through them becomes lower. During charging, the voltage of the storage cells 4 can rise to a maximum value designated B. As can be seen by reference to the graph of Figure 4, only a very low negative current Im will then flow through the storage cell. This means that additional storage units connected in series with the aforementioned unit but not charged completely to their maximum capacity, cannot receive any further 60 charge. In other words, this means that in a battery of a plurality of storage units the unit with the lowest capacity defines the total capacity of all storage units.

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Complete charging of all storage units is made possible bythe connection of a circuit according to the invention to each unit forming the battery. The protective circuit illustrated in Figure 1 contains the earlier-mentioned protective element 1. The charging current 11 is conducted through the storage cells 4 until65 3 GB 2 064 895 A 3 these have been charged to their maximum charging voltage. The protective element of the circuit is constructed so that if functions as a resistance until the said maximum charging current is reached, so that no current flows through the said protective element. When the storage cells have reached their maximum charging voltage, more particularly the voltage designed with B in Figure 4, the protective element 1 absorbs 5 the current and in this way bridges the circuit through the storage cells 4.

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The charging current In, which then flows through the protective element 1 which is constructed as a vessel, results in constant decomposition and recombination of the sodium hydroxide melt contained in the vessel, and heat is also generated. The chemical reactions take place in accordance with the following equations:

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Decomposition: 4 NaOH--> 4Na + 2H20 + 02 Recombination: 2 Na + 2H20 --).2 NaOH + OZ + HZ H2 + 1/202 ---> H20 15 Small quantities of catalyst such as platinum asbestos or Raney nickel can be added to assist the last recombination step.

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Since the charging current Im which flows through the protective element 1 is relatively low, it follows that the generated heat is also relatively low so that the switch 2, which is connected to the protective element via 20 the thermally conductive connection 3, is not actuated. Operation of the protective element 1 therefore provides precise information for the moment of time at which the storage unit is completely charged. By connecting an indicating instrument to the protective element it is possible to detect and visually indicate this moment of time. By means of the circuit according to the invention, the circuit of the already charged storage unit is bridged until all storage units of the battery are charged to their maximum capacity.

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The point of intersection with the ordinate U of the curve 1 plotted in Figure 4, characterizing the prevailing state of charge of the storage cell 4, is referred to as the quiescent voltage R of the storage cell. If the storage cells 4 have the quiescent voltage R indicated in Figure 4 or if a load is applied to the battery which has just been charged, the voltage of the storage cells 4 will again drop below the operating voltage of the protective element 1. Bridging of the circuit through the storage cells 4 is thus cancelled. Current flow between the electric connecting terminals 5 and 6 again takes place via the storage cells 4.

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If such a group of storage cells having the quiescent voltage R is continuously loaded, a discharge current will flow and the voltage of the storage cells 4 will diminish until it reaches zero and a short circuit current flows. If the group of storage cells connected in series with the aforementioned group are not fully discharged at that time, a discharge current la, illustrated in Figure 4, will be imposed from the outside on the 35 already discharged group. This current results in plurality reversal of the cell voltage, i.e. the latter acts negatively and can for example reach the value designated with C in the diagram. As already mentioned, the protective element of the protective circuit is constructed so that it again comes into operation and bridges the circuit through the storage cells when a predefinable discharge voltage thereof is reached. The discharging current la which flows through the protective element and is externally imposed, is relatively 40 large. Decomposition and recombination of the sodium hydroxide melt therefore takes place within the protective element. Simultaneously, a relatively large quantity of heat is generated. This is supplied via the thermally conductive connection 3 to the switch 2. The latter is constructed as a thermal switch and responds to the heat supplied to it. Its electrical contact is detached from the contact 8 and is connected to the contact 9. The storage cells are thus completely isolated from the circuit and are protected against destruction. 45 Complete discharge of the remaining and partially charged storage cells 4 of the battery can then be continued without interruption.

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In order to detach the electrically conductive contact of the switch 2 from the contact 8, the heat by the protective element 1 is supplied via the conductor 7, for example to a bimetal element contained in the switch and adapted to control the electrically conductive contact of the switch 2. Other thermally controlled 50 elements for operating the electric contact could also be used. The connection of the switch 2 to the contact 9 can be limited in time or can be continuous. The protective circuit can be constructed so that the connection of the switch 2 to the contact 9 is irreversible. This is the case if the electrically conductive contact of the switch is controlled by means of a fusible cutout. This means, that the endangered group of storage cells can be again rendered operational only by appropriate steps being taken in the workshop, after all storage cells 55 have been tested and recharged. The circuit can however also be constructed so that the switch 2 is connected to the contact 9 only for a finite time, i.e. a brief overdischarge is remedied by reducing the current by an interval and by recharging.

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Claims

CLAIMS .CLME:

1. A protective circuit for an electrochemical storage unit, which unit comprises an electrochemical storage cell or a plurality of such cells arranged in parallel and is connected in series with other such units to form a battery, wherein the protective circuit provides a protective element connected in parallel to said storage unit to by-pass said unit in response to excessive charging or discharging of the unit, wherein said 65 4 G B

2 064 895 A protective element comprises a vessel filled with a decomposable and readily recombinable chemical compound, and an electrically conductive member extending into said vessel by an aperture in the vessel, and wherein the external boundary surface of said vessel forms an anode and said conductive member forms a cathode, the vessel and the conductive member being electrically insulated from one another. 5 2. A circuit as claimed in claim 1, wherein the chemical compound is a sodium hydroxide melt.

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3. A circuit as claimed in claim 1 or claim 2, wherein the external boundary surface of the vessel comprises stainless steel.

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4. A circuit as claimed in any preceding claim, wherein the external boundary surface of the vessel comprises nickel.

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5. A circuit as claimed in any preceding claim, wherein the vessel is constructed as a totally enclosed 10 sleeve having at least one aperture for said conductive member.

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6. A circuit as claimed in any preceding claim, wherein the vessel is connected to a switch which connects one electrode of the storage unit to a positive or negative terminal.

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7. A circuit as claimed in claim 6, wherein the vessel is connected to the switch via a thermally conductive connection. 15

8. A circuit as claimed in any preceding claim, wherein the conductive member comprises a corrosion resistant metal. -

9. A circuit as claimed in any preceding claim, wherein the conductive member comprises copper.

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10. A circuit as claimed in any preceding claim, wherein the cell or cells of the storage unit are based on alkali metal and chalcogen with at least one anode chamber for the reception of the anoliyte and one cathode 20 chamber for the reception of the catholyte; said chambers being separated from each other by an alkali iron conducting solid electrolyte wall.

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11. A protective circuit for an electrochemical storage unit substantially as hereinbefore described with reference to and as illustrated in Figures 1 to 3 of the accompanying drawings.

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4 Printed for Her Majesty's Stationary Office, by Croydon Printing Company Limited, Croydon, Surrey, 1987. Published by The Patent office, 25 Southampton Buildings. London. WC2A 1AY, from which copies may be obtained.

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